For measuring the ripeness of a fruit without damaging the fruit, a method of measuring the change of the color of the fruit and a method of measuring the hardness of the surface of the fruit are tried. However, the former can not be applied for a kiwifruit, a pear and the like whose color does not change in the process of ripening, and, in the latter, there is big discrepancy between the measured result and the actual ripeness of the fruit because the hardness of the surface of the fruit does not represent the hardness of the flesh of the fruit.
Also, a method for measuring the sugar content of a fruit by using the technology of the spectral diffraction of near infrared rays is launched for practical use in sorting fruits such as melons, peaches and the like by ripeness. However, in this method, a just ripe fruit and a rotten fruit can not be discriminated because both have similarly high content of sugar, though the sugar content increases in the process of changing from unripe to ripe.
In the quality of a fruit, texture is an important factor as well as taste. In the United States, the judgment of the texture of an apple is performed by the panel test of experienced judging members. However, in this method, an objective judgment of the texture is hardly obtained because the judgments of the respective members are subject to the respective experiences and the likes of the members.
In plant physiology, it is known that the texture, i.e., the physical characteristics such as hardness, resistance to teeth, glutinousness and the like of a fruit relate to the viscosity value and the elasticity value of the fruit.
The softening of a fruit is caused by the change of the characteristics of the viscosity and the elasticity of the cell wall of the fruit. The mechanical structure of the cell wall of a fruit is maintained in such a manner that bundles of crystalline cellulose surrounded respectively by highly polymerized polysaccharides (i.e. matrix gel) are united by winding each other. The softening of a fruit is caused by the loosening of the winding of the bundles of the crystalline cellulose, which is caused by the low polymerization of the matrix gel. The polysaccharides which compose the matrix gel comprise various substances, and the substance which is low polymerized in the softening is different depending on the kind of fruit. As a result, the state of the matrix gel of a ripe fruit is different depending on the kind of fruit. The rough taste in the flesh of an apple or the like and the thick taste in the flesh of a berry such as kiwifruit or the like are caused by the different state of the matrix gel of each. The state of the matrix gel of a fruit represents the glutinousness of the fruit. In other words, the viscosity value of a fruit represents the feeling in eating such as the glutinousness, juiciness and the like of the fruit. Also the elasticity value of a fruit represents the elasticity force of the cell wall of the fruit, i.e. the hardness of the flesh of the fruit.
B. Drake designed a viscosity measuring apparatus for objectively measuring the texture of foodstuffs such as fruits by utilizing the fact that the texture can be shown by the characteristics of the viscosity and the elasticity of the foodstuffs (J. Food Sci. 27. P182-188:1962). The method is to compute the damping ratio .eta. of the foodstuff to be measured by an equation .eta.=.DELTA.f/f.sub.0, in which f.sub.0 is the intrinsic resonance frequency of the foodstuff, which is obtained by measuring the rectangular slice of the foodstuff, on which a vibration is applied, and .DELTA.f is a half-power bandwidth. And the damping ratio .eta. thus computed is deemed as the viscosity value of the foodstuff.
However, in this method also, the foodstuff to be measured is damaged because the rectangular slice has to be taken from the foodstuff for the measurement.